ergonomic

My back is killing me and it is mainly down to em slumping in my chair whilst using my computer....I have tried using an exercise ball to sit on and this seems to help but is not totally practical. I have seen kneeling stools / seats and have come across this (which seems expensive for what...

Does anyone know where in the net I can find an all leather ergonomic steering wheel with airbag for a 98 clk? Thanks